“The simplest hypothesis is that 3I/ATLAS is a comet,” Avi Loeb allows, “but let us instead maintain our childhood curiosity and seek evidence.” It has been amply rewarded in recent weeks, as a unprecedented set of observations has shown a phenomenon that has been observed on no other interstellar object in history: a non-uniform, high-altitude, Sun-pointing jet that is nearly exactly aligned with the comet’s axis of rotation.
Identified in a 37-night survey conducted at the Two-meter Twin Telescope facility in the Teide Observatory, the jet originated in an high-latitude active region on the illuminated hemisphere. Using image filtering techniques involving the Laplacian, which accentuates anisotropic detail in faint images against the brightness of the coma, it appears that there is an extended collimated jet pointing towards the Sun, whose position angle varies in a periodic fashion every 7.74 ± 0.35 hours, corresponding to a complete rotation period of 15.48 ± 0.70 hours if the object is close to a rotation pole, slightly lower than previously measured photometric periods of ~16.8 hours.
The geometry is très beau, as the anti-tail jet is keyed off by less than eight degrees from the rotation axis. Loeb finds that there is only a 0.5% chance of such a coincidental alignment. In this way, the comet held a steady dayside and nightside, switching roles at perihelion. Had this not been the case, the jet angle would have been much more erratic, with regions of quiescence as the comet source was turned largely towards the dark side. Nevertheless, the Sunward jet remained active beyond perihelion, suggesting a secondary region of volatiles near the opposite pole.
The size of the jet body is huge, with estimated structures from the anti-tail measured to be as large as 1 million kilometers. The precession and persistence of the jet indicate a non-surface process but a localized release of gas. Computer simulations based on the coma topography can accurately match the observed fan shape formed by micron-sized dust particles with ejection velocities predicted to be a result of CO2 gas drag forces.
Spectroscopic studies added to the enigma. With the NIRSpec IFU instrument of the new telescope, the composition of the coma was determined to be dominated by CO2, along with a CO2:H2O ratio of 8.0±1.0, which was the highest found in any comet and 6.1σ above the trend for Solar System comets orbiting at similar heliocentric distances. Besides these, the spectrum was also found to indicate the presence of carbon monoxide, OCS, water ice, and dust, but the abundance of water vapor was found to be abnormally low.
These findings indicate that the origin of the comet could be the formation of the coma material close to the ice line of CO2 in the protoplanetary disk from which the solar system evolved, or the material may be the result of the comet’s prolonged irradiation by the
This dust and gas morphology confirms the results of the spectrum. The increased scattering of the 1.2 μm sunward-looking particle trail reflects the outgassing of CO₂, which means that the main cause of the dust egress is the sublimation of CO₂. Large compact particles are dominant, as indicated by the polarimetric observations. This confirms the low effect of solar radiation pressure as well as the smooth transition between the sunward fan and the anti-solar tail.
From dynamical considerations, 3I/ATLAS has a hyperbolic trajectory at an inbound velocity of 57.95+−0.05 km/s, as would be expected of an origin in the Galactic thin disk. The dynamical signature indicates that this star simulates the movement of a star like our star, but with low metal content, and it indicates dynamical ages of 3 to 11 billion years. The visual spectrum of the comet is redder than D-type asteroids but visually similar to TNOs, with a slope of 18.3+−0.9 %/1000 Å.
Loeb has hypothesized that it is even possible that the exact alignment of the jets may be intended if the object is a technological spacecraft, with the thrusters pointing towards the Sun in order to protect the craft from the damage caused by micrometeorites. NASA’s Tom Statler has a different explanation and argues that it looks like a comet. It does comet things, and that comets have weird properties in the first place, particularly the ones which originated from outside our Solar System.
Irrespective of their origin, the discovery of this wobbling, sunward-pointed jet in 3I/ATLAS presents an unprecedented opportunity for the community to study the physics of interstellar cometary activity, states KAVLI/CNES researcher Woodward. The intersection of precise orbital alignment, highly enriched CO2, and morphology stability pushes the bounds of our understanding and shows that multi-instrument and multiwavelength efforts are essential in preserving the transient characteristics of such brief interstellar visitors before they return to the interstellar medium.
